Detailed observations of Saturn's moon Titan have now spanned 30 years, covering an entire solar orbit for this distant world. Dr Athena Coustenis from the Paris-Meudon Observatory in France has analysed data gathered over this time and has found that the changing seasons of Titan affect it more than previously thought. Dr Coustenis presented these results at the European Planetary Science Congress in Madrid on Friday 28th September.

"As with Earth, conditions on Titan change with its seasons," says Coustenis. "We can see differences in atmospheric temperatures, chemical composition and circulation patterns, especially at the poles. For example, hydrocarbon lakes form around the north polar region during winter due to colder temperatures and condensation. Also, a haze layer surrounding Titan at the northern pole is significantly reduced during the equinox because of the atmospheric circulation patterns. This is all very surprising because we didn't expect to find any such rapid changes, especially in the deeper layers of the atmosphere."

The main cause of these cycles is solar radiation. This is the dominant energy source for Titan's atmosphere, breaking up the nitrogen and methane present to create more complex molecules, such as ethane, and acting as the driving force for chemical changes. Titan is inclined at around 27 degrees, similar to the Earth, meaning that the cause of seasons – sunlight reaching different areas with varying intensity due to the tilt – is the same for both worlds.

"It's amazing to think that the Sun still dominates over other energy sources even as far out as Titan, over 1.5 billion kilometres from us," explains Coustenis. To draw these conclusions data was analysed from several different missions, including Voyager 1 (1980), the Infrared Space Observatory (1997), and Cassini (2004 onwards), complemented by ground-based observations. Each season on Titan spans around 7.5 years, while it takes 29.5 years for Saturn to orbit the Sun, so data has now been gathered for an entire Titan year, encapsulating all seasons.

"Titan is the best opportunity we have to study conditions very similar to our own planet in terms of climate, meteorology and astrobiology and at the same time a unique world on its own, a paradise for exploring new geological, atmospheric and internal processes," concludes Coustenis.

Evidence of changing weather patterns in the skies over Titan's southern region are revealed in the false color images at the top of the page obtained by the Cassini spacecraft's visual infrared mapping spectrometer over flybys of this largest of Saturn's satellites. In the first image (left), obtained on the Oct, 26, 2004 Titan flyby, from a distance of some 200,000 kilometers (124,300 miles), Titan's skies are cloud-free, except for a patch of clouds observed over the south pole near the bottom of the image.

In contrast, the image on the right shows a recent view of this same area of Titan obtained seven weeks later on the second close Titan flyby on Dec. 13, 2004, from a distance of 225,000 kilometers (139,800 miles). This image clearly shows that several extensive patches of clouds have formed over temperate latitudes. The appearance of these clouds reveals the existence of weather.

Tracking these features is currently underway by scientists, who hope to gain a better understanding of global circulation, regional weather patterns, and localized meteorology in Titan's skies.

The colors red, green, and blue represent near-infrared images obtained at 2.01 micron, 2.83 micron and 2.13 micron, respectively. These colors explore the surface and atmosphere of Titan with varying effectiveness. The red color images the surface at a wavelength (2.01 micron) where the surface is relatively bright, making the surface appear reddish in these color images. The green color (2.83 micron) images the surface as well, but due to enhanced absorption of sunlight by the surface and lower atmosphere, the surface is relatively dark here compared to the red. The blue color (2.13 micron) is at a wavelength where sunlight cannot reach the surface at all due to strong absorption by the atmospheric gas methane.

In contrast to the reddish surface, bright clouds at a relatively high altitude (here, about 30 kilometers (19 miles) above the ground) residing above most of the atmospheric absorption appear whitish in these representations, as they reflect sunlight effectively in all three near-infrared colors.